Cubic boron nitride is second only to diamond in hardness but has a greater chemical stability, and is therefore becoming increasingly more important as a grinding, polishing and cutting material.
U.S. Pat. No. 4,188,194 describes a process for making sintered polycrystalline CBN compacts which utilizes pyrolytic boron nitride (PBN) in the absence of any catalyst. An improvement on this direct conversion process is disclosed in U.S. Pat. No. 4,289,503 wherein boric oxide is initially removed from the surface of the HBN powder.
A variety of methods have been proposed and used in the production of CBN, but the most well-known of these, which is widely used commercially, is a method of converting HBN to CBN under the high-temperature/high-pressure (HT/HP) conditions of about 50-75 Kbar and 1400.degree.-1600 .degree. C. in the presence of a catalyst.
Typical catalysts for the HP/HT process have conventionally been nitrides, amides, fluorides of alkaline and alkaline earth elements, urea, aluminum nitride, Fe-Al and Mg-Al alloys, nitrides and boronitrides of alkali metals and alkaline earth metals. Of these, lithium-type catalysts have been thoroughly studied, and lithium nitride and lithium boronitride are considered to be particularly effective catalysts (see, for example, U.S. Pat. No. 3,772,428).
One method of converting HBN to CBN employs at least one catalyst selected from the class consisting of alkali metals, alkaline earth metals, lead, antimony, tin and nitrides of these metals as described by Wentorf Jr. in U.S. Pat. No. 2,947,617, incorporated herein by reference.
Another method employing Fe.sub.3 Al and certain silver-cadmium alloys as catalysts in the conversion of HBN to CBN has been described in "Synthesis of Cubic Boron Nitride" by Saito et al. (Yogyo-Kyokai Shi, Vol. 78, No. 893).
Another method employs aluminum alloys of cobalt, nickel and manganese as catalysts for the conversion of HBN to CBN at HP/HT conditions. The habit and size of the CBN crystals developed depends on whether HBN or a mixture of HBN and CBN crystals (or sintered carbide grains) is employed with the catalyst metal. This method is described by Wentof, Jr., et al. in U.S. Pat. No. 3,918,219, incorporated herein by reference.
Even with the use of catalysts, the pressures and temperatures required for CBN growth are high, e.g., 50-75 Kbar and 1400.degree.-1800 .degree. C., which necessitates the use of a belt type HP/HT apparatus such as described in U.S. Pat. Nos. 4,409,193, 4,810,479 and 2,947,617 and French Pat. No. 2,597,087.
CBN can also be grown at lower pressures, approximately 30 Kbar, using hydrazine (NH.sub.2 NH.sub.2) as a catalyst. However, use of this catalyst is not practical because hydrazine is a carcinogen, is highly corrosive, flammable, and reactive, and is potentially explosive when it comes in contact with air.
CBN cutting and abrasive tools are typically in the form of compacts. A compact is a mass of abrasive particles bonded together in a self-bonded relationship (see U.S. Pat. Nos. 3,852,078 and 3,876,751); by means of a bonding medium (see U.S. Pat. Nos. 3,136,615, 3,233,988, 3,743,489, 3,767,371, and 3,918,931); or by means of combinations thereof. A composite compact is a compact bonded to a substrate material, such as cemented metal carbide. Compacts or composite compacts may be used in blanks for cutting tools, drill bits, dressing tools, and wear parts (see U.S. Pat. Nos. 3,136,615 and 3,233,988).
Sintered CBN/cermet compacts prepared by HP/HT pressing of mixtures of CBN, various cermets, and aluminum compounds (with or without metal additions) are disclosed in U.S. Pat. Nos. 4,334,928 and 4,389,465. U.S. Pat. No. 4,394,170 proposes a composite sintered compact containing high density BN (CBN and Wurtzite boron nitride, WBN), ceramic and various metal bases obtained by HP/HT processing of mixtures of CBN, WBN, and various ceramic and metal mixtures. A supported version of such CBN/cermet compact is disclosed in U.S. Pat. No. 4,403,015. These prior CBN cermet compact disclosures, however, must initially make the CBN in one HP/HT step and the CBN cermet product in a second HP/HT step. This necessarily results in extra wear on the HP/HT presses, reduced throughput, and increased manufacturing costs result from the use of two different HP/HT processing operations.
The above-mentioned catalysts and processes all require high temperature and high pressure to obtain CBN in sufficient yields and therefore are less than desirable from an industrial point of view. Also, the CBN obtained by use of said catalysts have irregular shapes or nearly spherical shapes, poor in development of euhedral planes.
It is an object of the present invention, in the light of these circumstances, to provide a method of converting HBN to CBN with a high yield, using lower temperatures and pressures than are currently practised.
It is a further object of the present invention to provide a method of converting HBN to CBN in high yield using temperatures and pressures lower than the prior art through the use of a stable and relatively inexpensive catalyst.
These and other objects of the invention are accomplished in a standard HP/HT process by using melamine (C.sub.3 H.sub.6 N.sub.6) as a catalyst.